1. Field of the Invention
The invention relates to a corrosion-inhibiting material comprising one or more volatile corrosion inhibitors evenly distributed in a metal oxide gel matrix. The metal oxide gel controls the release of volatilized inhibitor.
2. Background
It is known that corrosion inhibitors having a tendency to sublimate under normal conditions in the powder form and being able to gain access to the metal surfaces to be protected via the gas phase may be put to use for temporary corrosion protection of metal articles within confined environments, e.g. in packages or show cases. These so-called vapor phase corrosion inhibitors (VPI) or volatile corrosion inhibitors (VCI) are typically employed as a volatile powder, packaged in bags of a vapor phase-permeable material.
The present invention can use as a VCI, any known volatile corrosion inhibitor or inhibitors. These are exemplified in the discussion of the prior art presented below, the disclosures of various VCI being incorporated herein by reference.
Variants of volatile corrosion inhibitors (VCI) are known e.g. from H. H. Uhlig "Corrosion and Corrosion Protection" (German), Akademie-Verlag Berlin, 1970, page 247 et.seq. or from I. L. Rozenfeld "Corrosion Inhibitors" (Russian), Izt-vo Chimija Moskva 1977, page 316 et. seq. Their drawback is that VPI release occurs in an undefined manner and no homogenous distribution throughout the gas environment can be assured. Further disadvantages include the risk of the bag containing the VCI being mechanically ruptured, resulting in undesirable contamination of the packaged articles as well as in problems resulting from the irregular distribution of the bags in large-area storage rooms and large containers.
Attempts to obviate these disadvantages by many ways and means, have been described. U.S. Pat. No. 3,836,077 proposes employing a VCI mixture in the form of compressed pellets and either to avoid completely a gas-permeable container material or to make use of the pellets embedded in foamed materials provided with suitable cavities. By contrast, U.S. Pat. Nos. 3,967,926; 5,332,525 and 5,393,457 propose mixing the VCI's with a chemically inert powder or a drying agent such as silica gel or zeolite. This allows use of tougher, air-permeable plastic films or capsules to replace bags made of natural products (cotton, linen, etc) which were used earlier with the intention that the inert substrate material contributes, by its porous structure, to continual sublimation of the VCI components distributed therebetween. The drying agent was used to counteract an agglomeration of the finely dispersed VCI components into larger mixed particles (e.g. clumping with a crusted surface due to water being absorbed). However, in practice, the use of drying agents normally results in the opposite of the desired effect and leads to clumping following water absorption. In addition, the mechanically more stable container materials have a lesser permeability to the VCI vapor than the natural products so that their emission rate is reduced, this being the reason why a larger number of VCI reservoirs are needed than when using containers of natural products for controlling the level of VCI vapor concentration necessary for corrosion protection. This drawback further complicates and makes temporary corrosion protection, especially in spacious interiors, even more expensive.
For eliminating the complicated step of assuring homogenous distribution of VCI reservoirs in the interiors of packages commensurate with automated packaging systems, attempts have been made to suitably fix the VCI to the packaging material, these attempts initially being dominated naturally by paperboards and packing papers. To ensure directed emission of the released vapors of the applied VCI into the interiors the VCI components are usually applied to only one side of the packaging materials while the other side later arranged as the outer front side, receives a protective lacquer coating which is inherently water-repellant and may also act as a vapor barrier for the VCI existing on the reverse side (cf. e.g. H. H. Uhlig, loc cit). The problem still existing today is fixing the VCI to the surface of paperboard or packing paper to be stable in dimension and quantity. If the VCI is applied within an organic coating material, many substances effective as VCI cannot be put to use since they enter into a chemical reaction with the binding agent of the coating material, becoming trapped in the resulting polymer matrix and are no longer capable of sublimation. This drawback is evident in the case of e.g. VCI's embedded in acrylate, alkyd, epoxide or phenolic resin-based polymer binding agents.
As an alternative the VCI is dissolved in an organic solvent with which the packaging material is soaked. Methods of this kind involving various active substances and solvents are described e.g. in JP 61-227188, JP 62-063686, JP 63-02888, JP 63-183182, JP 63-210285 and U.S. Pat. No. 3,887,481. However, these all have the disadvantage that after evaporation of the solvent the VCI is present within the pores of the corresponding substrate in the form of fine crystals which adhere to the packaging material only slightly. There is therefore the risk of these active substances becoming dissociated from the packaging material and thus there is no assurance that the paperboards and papers pretreated therewith exhibit the necessary specific surface concentration of VCI at the time of their use for corrosion protection.
To confine this drawback at least in its extent it is proposed in DE 9210805 to prepare only one ply of the corrugated paperboard as the substrate and depot for the sublimable corrosion inhibitors and to cover this ply on both sides by at least one further porous ply so that the VCI deposit is located in the interior of the paperboard. Since this hampers VCI emission into the interior of the package it is proposed in JP 4 083 943 to use instead of corrugated paperboard or paper an expanded polyurethane having a substantially higher porosity and is thus able to absorb much larger quantities of VCI. However, the disadvantage here is that after evaporation of the solvent the VCI is present in the pores of the foamed material as crystals with less tack so that the VCI may easily bleed uncontrolled should the packaging material be ruptured.
JP 58-063732 and U.S. Pat. No. 4,275,835 thus specify methods in which the VCI is a component of the foamed polymer, this making it necessary that the crystalline VCI is dispersed in one of the starting components. Despite highly complicated and energetic methods this is possible to only a limited extent since VCI usually belong to other classes of substances, as a result of which the stability is low. These methods are further aggravated as modern VCI's themselves comprise several substances having differing chemical properties and thus, as far as these can be dispersed at all together with the components for expanded materials, such dispersions usually have a very broad grain size spectrum, low stability and are problematic in processing.
DD 295 668 specifies a method of producing polyurethane systems containing VCI in which the VCI are first dissolved in a multifunctional alcohol having the mol mass 500 to 1000 g/mol and are subsequently introduced into the polyol before the polyurethane is generated by the addition of polyisocyanate, a catalyst, stabilizer and an expanding agent. This method is, however, restricted only to VCI which are soluble in alcohols having the necessary concentration for the corrosion protection while not being detrimental to the expansion process as a constituent of the polyol component. This method is thus not suitable to satisfy the complex requirements made nowadays on temporary corrosion protection of ferrous and non-ferrous metals as well as on multi-metal combinations, since it excludes practically all inorganic active substances from the application.
To avoid these drawbacks and to provide VCI-vapor emitting packaging material suitable for application in modern packaging, storage and transport systems it is proposed in U.S. Pat. No. 4,124,549; U.S. Pat. No. 4,290,912; U.S. Pat. No. 5,209,869; EP 0 639 657 and DE-OS 3 545 473 to introduce the VCI during extrusion of films of polyolefines so that a physically stable polymer packaging material results from which the VCI are emitted. EP 0 662 527, DE-OS 4 040 586, DE OS 3 518 625 and U.S. Pat. No. 5,139,700 propose as a further sophistication employing such a polyethylene or polypropylene-based film containing VCI only in conjunction with laminated multi-ply materials, whereby one ply oriented outwards consists of an Al foil or a film of polymer densely cross-linked functioning as a vapor barrier as regards the active substances emitted from the ply containing the VCI and prompting directed transport of VCI into the interior of the packaging material. Producing polymer films containing an inhibitor by extruding a blend containing substances tending to sublimate is naturally thwart with difficulties: (a) the high volatility of VCI at temperatures at which the extrusion process is undertaken results in significant losses of these substances as well as to expansion of the film, impairing their closed configuration and thus to an uncontrolled reduction in their strength and protective properties, (b) there is a possibility of thermal decomposition of the corrosion inhibitors and undesirable thermochemical reactions with the polymer matrix. The serious disadvantage resulting therefrom is that it is hardly possible in this way to produce a packaging material having reproducible, uniform surface properties.
The object of the invention is to provide an improved material for fixing vapor phase or volatile corrosion inhibitors mechanically and chemically stable to solid surfaces and a corrosion-protective packaging material. The fixing material is intended to permit universal and technically simple application, more particularly independently of the physical and chemical properties of the active substances and the nature of the substrate surface while obviating the drawbacks of the methods as described above. It is furthermore an object of the invention to define a method for producing such a material.